Metal-to-metal charge transfer driven by electropositive species for stable anionic redox

Abstract

Apart from conventional redox chemistries, exploring high-voltage redox processes, such as high-valent redox of a transition metal (TM) or novel anionic redox, is challenging due to unstable energy states correlated with destructive structural disorder in intercalation-type cathode materials. Here, we show a new strategy to design high-energy-density 4d-based Li-excess oxides with strong structural reversibility by substituting electropositive species. It is known that TMs in 4d-based Li-excess oxides, in contrast to 3d-based oxides, actively interact with neighboring oxygen ligands, inducing b1/b1* energy levels. Metal-to-metal charge transfer, driven by covalency competition within the asymmetric TM3d-O-TM4d backbone, induces a larger overlap between electronegative species and oxygen ligands, leading to stable high-voltage redox. Furthermore, we reveal that redox-inactive dopants control the reversibility of cation migration, thereby extending the battery lifetime. These insights open new perspectives for the control of intrinsic redox chemistry and enable rational designs for high-energy-density Li-excess cathodes.